dt-bindings: iio: adc: ti,adc12138 yaml conversion.

Simple binding conversion from txt to yaml.
Only addition was #io-channel-cells to allow for potential consumers
of the channels on this device.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Link: https://lore.kernel.org/r/20200830161154.3201-2-jic23@kernel.org